Not just a good circle. A great circle.

I assume you know that the shortest distance between two points on a sphere is a curved line. If you don’t then, you are probably not going to be amused by the title of this post.

Back to what I was saying, the shortest distance between two points on a sphere is a straight line. Usually that doesn’t matter to us because we are not spanning very much of the sphere we live on. However, when you fly, it matters. There is a really neat website which maps the shortest distance between airports.

To get a sense of when great circle navigation matters, look at these three maps: Phoenix to L.A. looks fairly straight, and it is. Phoenix to Vancouver starts to show a slight curve, but not a lot. Phoenix to Tokyo looks like a good part of a circle.

Now look at Phoenix to Hong Kong. You can really see the great circle and now you have to wonder why there is not a straighter line. Grab a basketball or other sphere. Stretch a string between two points about half-way around the ball, and you’ll find when you pull it tight it makes that same approximate curve or great circle. You actually cross over the Aleutian Islands in Alaska if you are heading from Phoenix to Hong Kong. Go figure!

Have a great, not just a good, day!

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